The 3rd Generation Partnership Project (3GPP) is a collaboration agreement that was established in December 1998. The collaboration agreement brings together a number of telecommunications standards bodies to produce global specifications and technical reports for a 3rd Generation mobile communications system. High-speed Downlink Packet Access (HSDPA) is a feature that was introduced in Release 5 of the 3GPP specification. HSPDA achieves maximum spectral efficiency using three concepts: Adaptive Modulation and Coding (AMC), fast physical layer retransmissions (Hybrid ARQ), and fast Node B scheduling.
FIG. 1 is a diagram of a system 100 configured to utilize HSPDA. The system 100 includes a core network 102 and a radio network controller (RNC) 104 that communicates with the core network 102. A plurality of Node Bs 106 communicate with the RNC (for clarity, only two Node Bs are shown in FIG. 1; it is understood that many more Node Bs can be in communication with a single RNC). Each Node B 106 controls a plurality of cells 108 (again, the number of cells 108 shown in FIG. 1 is only exemplary). A wireless transmit/receive unit (WTRU) 110 can communicate with one or more of the cells 108.
AMC adapts the transmission data rate on the High Speed Downlink Shared Channel (HS-DSCH) according to the channel conditions perceived by the WTRU. Specifically, a Node B determines the best data rate, coding, and transport block size using the following information obtained from the WTRU on the High Speed Dedicated Physical Control Channel (HS-DPCCH):
1. Channel Quality Indication (CQI), which indicates the channel conditions as monitored by the WTRU, and
2. Acknowledge/negative acknowledge (ACK/NACK) feedback used for fast retransmissions (HARQ).
Handover is the process in which a WTRU switches from one cell to another cell without service interruption. In HSDPA, the high-speed shared channels are monitored by the WTRU in a single cell, which is called the “serving HS-DSCH cell”. When a handover occurs, the WTRU needs to switch to a new serving HS-DSCH cell (the target cell/Node B) and stop communication with the old serving HS-DSCH cell (the source cell/Node B). This procedure is also called a serving HS-DSCH cell change.
FIG. 2 shows a flowchart of a method 200 for performing a handover procedure. The WTRU continuously measures the signal strength of neighboring cells (step 202). The WTRU compares the measured signal strength of the neighboring cells with the strength of the signal from the serving cell (step 204). Once the measured signal strength on the monitored common pilot channel (CPICH) of the neighboring cell exceeds the signal strength of the serving cell, the WTRU indicates to the RNC that a cell change is needed (referred to as “a change of best cell”; step 206). The change of best cell is reported from the WTRU to the RNC via an RRC MEASUREMENT REPORT event 1D. This report contains the measured signal strength value and the cell ID. The RNC then makes the final decision whether a serving HS-DSCH cell change is going to occur. The handover is then executed, to switch the WTRU to the new Node B (step 208).
The new serving HS-DSCH cell needs to start downlink transmission to the WTRU at the time the channel configuration takes place (step 210). In order to achieve a maximum transmission rate and maximum spectral efficiency, the Node B needs to adapt to the new channel conditions as fast as possible. The channel quality conditions measured by the WTRU in the new cell are not known by the target Node B, therefore a maximum Modulation Coding Scheme (MCS) cannot be achieved right away. The Node B has to wait to receive a few CQI reports from the WTRU before transmitting at an optimal data rate.
Prior to the CQI reports being transmitted to the Node B, the target Node B may start transmitting at a lower rate than the WTRU can support. This would waste HSDPA resources until the Node B can adapt to the new channel conditions. Keeping in mind that the RNC performed the Node B switch because the downlink channel conditions are better in the new cell, the cell change could be a waste of capacity and resources.
On the other hand, prior to the appropriate number of CQI reports being received by the target Node B, the target Node B may assume that the new channel conditions are better than in the old cell and start transmission at a higher data rate to avoid wasting HSDPA resources. However, the WTRU could be measuring unfavorable channel conditions at the instant after handover, and might have trouble decoding the data over the HS-DSCH. Such trouble would trigger re-transmissions and higher error rates until the Node B adapts to the new channel conditions.
Existing 3GPP Release 6 specifications do not provide support for optimal MCS selection and scheduling for the HS-DSCH in the new serving cell after a handover occurs.